Stellar Mass Growth in the First Galaxies: Theory and Observation

We compare the growth in stellar mass of galaxies in the $6<z<12$ epoch with predictions of a semi-analytic galaxy formation model - Galacticus. In contrast to diverse and controversial results that compare models and data for the \emph{luminosity} evolution of galaxies – reported in an abundance of recent papers, we find very good, unambiguous agreement in the more fundamental quantity of stellar mass - measured from JWST observations - and Galacticus predictions. Specifically, we find good agreement for the shape of the integrated stellar mass as a function of redshift without any adjustment of parameters, and in \emph{amplitude} as well, when ‘feedback’ is lowered by a factor of 3 compared to that required to match later-universe models and data. This result emerged from detailed investigation of the claim by Dressler et al. that bursts of star formation dominated the growth in stellar mass, specifically, that half of the galaxies with stellar mass growth of $M_* > 2\times10^8 \mathrm{M}\odot$ in the epoch $8<z<6$ had less than $M*<\times10^8 \mathrm{M}_\odot$ prior to $z = 8$. Here too we find agreement between models and data, namely that these ~100 Myr ‘bursts’ had strong in situ growth at $z\le8$, or showed (in Galacticus) substantial stellar and/or gas-rich mergers, and 30-40 Myr ‘starbursts’ as are common in $z<3$ galaxies. We note that, if a theoretical simulation is unable to pass the test of matching the growth of stellar mass, any success in reproducing the luminosity function is meaningless.

💡 Research Summary

The authors compare the evolution of stellar mass in galaxies between redshifts 6 and 12 with predictions from the semi‑analytic model Galacticus. They argue that, unlike the many recent studies that focus on the luminosity function and often require ad‑hoc adjustments to reproduce the observed number of bright galaxies, the stellar mass—being a more fundamental quantity—matches the model predictions without any fine‑tuning. Using JWST photometry in seven near‑infrared bands, Dress et al. (2024) derived star‑formation histories (SFHs) for ∼ 1000 galaxies and classified them into burst‑like, continuous‑like, stochastic‑like and continuous‑over‑four‑epochs – the classification is based on the presence or absence of stellar mass in discrete redshift bins. The authors adopt a “burst” definition that compares the total stellar mass formed in the lower‑redshift interval 6–7 (≈ 40–200 Myr) with that formed in the higher‑redshift interval 8–12. They then apply the same definition to the Galacticus output. The key result is that the integrated stellar‑mass density as a function of redshift is reproduced by Galacticus when the feedback strength is reduced by a factor 3 relative to the values calibrated for lower‑redshift data. In the default Galacticus run the model under‑predicts the total stellar mass by a factor ≥ 4, but a modest weakening of the outflow parameters (V_outflow≈120 km s⁻¹, α_outflow≈3.0) brings the model into excellent agreement with the JWST measurements from z = 12 down to z = 6. The authors also examine the “burst‑fraction” reported by Dress et al., namely that roughly half of the galaxies with M⋆ > 2 × 10⁸ M⊙ at z ≈ 6–8 had M⋆ < 10⁸ M⊙ before z = 8. Galacticus reproduces this fraction (≈ 67 % in the weakened‑feedback run) and shows that the “burst” galaxies achieve their rapid mass growth either through intense in‑situ star formation or through major stellar or gas‑rich mergers. The paper emphasizes that a model that cannot match the stellar‑mass growth is of limited value, even if it reproduces the luminosity function, and suggests that future high‑redshift studies should prioritize mass‑based tests. The work also notes consistency with independent high‑resolution FIRE‑Box simulations and with the observed merger rates, and it highlights the sensitivity of early‑universe stellar‑mass assembly to the assumed feedback efficiency.